Fullerenes are spheroidal, closed-cage molecules consisting essentially of sp2-hybridized carbons typically arranged in hexagons and pentagons. Fullerenes, such as C60, also known as Buckminsterfullerene, more commonly, xe2x80x9cbuckyballs,xe2x80x9d and C70, have been produced from vaporized carbon at high temperature. Presence of a transition metal catalyst with the high temperature vaporized carbon results in the formation of single-wall tubular structures which may be sealed at one or both ends with a semifullerene dome. These carbon cylindrical structures, known as single-wall carbon nanotubes or, commonly, xe2x80x9cbuckytubesxe2x80x9d have extraordinary properties, including both electrical and thermal conductivity and high strength.
Nested single-wall carbon cylinders, known as multi-wall carbon nanotubes possess properties similar to the single-wall carbon nanotubes, however, single-wall carbon nanotubes have fewer defects, rendering them stronger, more conductive, and typically more useful than multi-wall carbon nanotubes of similar diameter. Single-wall carbon nanotubes are believed to be much more free of defects than are multi-wall carbon nanotubes because multi-wall carbon nanotubes can survive occasional defects by forming bridges between the unsaturated carbon of the neighboring cylinders, whereas single-wall carbon nanotubes have no neighboring walls for defect compensation.
In defining the size and conformation of single-wall carbon nanotubes, the system of nomenclature described by Dresselhaus, et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19, will be used. Single-wall tubular fullerenes are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When n=m, the resultant tube is said to be of the xe2x80x9carm-chairxe2x80x9d or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. When m=0, the resultant tube is said to be of the xe2x80x9czig zagxe2x80x9d or (n,0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig zag pattern. Where nxe2x89xa0m and mxe2x89xa00, the resulting tube has chirality. The electronic properties are dependent on the conformation, for example, arm-chair tubes are metallic and have extremely high electrical conductivity. Other tube types are metallic, semimetals or semi-conductors, depending on their conformation. Regardless of tube type, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
Several methods of synthesizing fullerenes have developed from the condensation of vaporized carbon at high temperature. Fullerenes, such as C60 and C70, may be prepared by carbon arc methods using vaporized carbon at high temperature. Carbon nanotubes have also been produced as one of the deposits on the cathode in carbon arc processes.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus by simultaneously evaporating carbon and a small percentage of Group VIIIb transition metal from the anode of the arc discharge apparatus. These techniques allow production of only a low yield of carbon nanotubes, and the population of carbon nanotubes exhibits significant variations in structure and size.
Another method of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms (such as nickel, cobalt, or a mixture thereof) to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be formed in clusters, termed xe2x80x9cropes,xe2x80x9d of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. Although the laser vaporization process produces an improved yield of single-wall carbon nanotubes, the product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials. In addition, the laser vaporization of carbon is a high energy process.
Another way to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then precipitate as part of a nanotube from one side of the particle. This procedure typically produces imperfect multi-walled carbon nanotubes.
Another method for production of single-wall carbon nanotubes involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 on alumina-supported transition metal particles comprising Mo, Fe, Ni, Co, or mixtures thereof. This method uses inexpensive feedstocks in a moderate temperature process. However, the yield is limited due to rapid surrounding of the catalyst particles by a dense tangle of single-wall carbon nanotubes, which acts as a barrier to diffusion of the feedstock gas to the catalyst surface, limiting further nanotube growth.
Control of ferrocene/benzene partial pressures and addition of thiophene as a catalyst promoter in an all gas phase process can produce single-wall carbon nanotubes. However, this method suffers from simultaneous production of multi-wall carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature conditions necessary to produce high quality single-wall carbon nanotubes.
More recently, a method for producing single-wall carbon nanotubes has been reported that uses high pressure CO as the carbon feedstock and a gaseous transition metal catalyst precursor as the catalyst. (xe2x80x9cGas Phase Nucleation and Growth of Single-Wall Carbon Nanotubes from High Pressure Carbon Monoxide,xe2x80x9d International Pat. Publ. WO 00/26138, published May 11, 2000 (xe2x80x9cWO 00/26138xe2x80x9d), incorporated by reference herein in its entirety). This method possesses many advantages over other earlier methods. For example, the method can be done continuously, and it has the potential for scale-up to produce commercial quantities of single-wall carbon nanotubes. Another significant advantage of this method is its effectiveness in making single-wall carbon nanotubes without simultaneously making multi-wall nanotubes. Furthermore, the method produces single-wall carbon nanotubes in relatively high purity, such that less than about 10 wt % of the carbon in the solid product is attributable to other carbon-containing species, which includes both graphitic and amorphous carbon.
All known processes for formation of single-wall nanotubes involve a transition-metal catalyst, residues of which are invariably present in the as-produced material. Likewise, these processes also entail production of varying amounts of carbon material that is not in the form of single-wall nanotubes. In the following, this non-nanotube carbon material is referred to as xe2x80x9camorphous carbon.xe2x80x9d
There are chemical processes involving single-wall carbon nanotube manipulation for specific applications, such as, for example, xe2x80x9cChemical Derivatization Of Single-Wall Carbon Nanotubes To Facilitate Solvation Thereof; And Use Of Derivatized Nanotubes,xe2x80x9d International Pat. Publ. WO 00/17101, published Mar. 30, 2000, and xe2x80x9cCarbon Fibers Formed From Single-Wall Carbon Nanotubes,xe2x80x9d International Pat. Publ. WO 98/39250, published Sep. 11, 1998, both of which are incorporated by reference herein. Many of these manipulation processes involve chemical reaction of the single-wall carbon nanotube sides and/or ends with other chemicals. These processes also often involve the physical interaction (through van der Waals or other inter-molecular forces) of nanotubes with one another or interaction of nanotubes with other matter within which they are suspended, encapsulated, or otherwise placed in proximity. Clearly, in performing a chemical or physical interaction process with nanotube material, any impurities present are likely to inhibit or modify such manipulation process and/or physical interactions making it difficult or even impossible to achieve the intended result.
One example of a nanotube interaction that has many uses is the process of self-assembly of nanotubes. Under some conditions, individual nanotubes self-assemble into xe2x80x9cropesxe2x80x9d of many parallel nanotubes in van der Waals contact with one another. See, e.g., xe2x80x9cMacroscopic Ordered Assembly of Carbon Nanotubes,xe2x80x9d International Pat. Publ. WO 01/30694 A1, published May 3, 2001, incorporated herein by reference. Likewise, individual single-wall carbon nanotube and ropes of single-wall carbon nanotube, can be caused to aggregate into large networks, which are themselves electrically conductive. This self-assembly process enables nanotubes and ropes of nanotubes to form such networks when they are suspended in a matrix of a different material. The presence of this network alters the electrical properties of a composite that includes nanotubes. The facility with which single-wall carbon nanotubes aggregate into ropes and networks is critically dependent upon the purity of the nanotube material.
Another example of nanotube manipulation is the chemical processing of nanotubes by reacting them with other chemicals to produce new materials and devices. Clearly, the presence of other species such as the transition metal catalyst or amorphous carbon material provides sites for chemical reaction processes that are distinct from the desired chemical reaction process involving the nanotube alone. As with any species involved in a chemical process, one seeks to perform that process with a pure species.
Likewise, the useful properties and behavior of nanotube-containing materials devices and articles of manufacture derive from the properties of the nanotubes themselves, and the absence of impurities in the nanotube material enhances the performance of any and all materials, devices, and articles of manufacture comprising nanotubes. Particular examples are those where the material, device, or article of manufacture must function in a high magnetic field or a chemically-active environment. Examples of such materials, device and articles would include those subjected to traditional nuclear magnetic resonance apparatus; those serving as electrodes in batteries, capacitors, sensors, and fuel cells; those implanted in or otherwise in contact with any living organism; those used in preparation of other materials requiring low-contamination environments (such as chemical apparatus, chemical storage devices, electronic materials and devices, or food processing equipment).
In environments where there are chemicals, such as oxygen, that react with the nanotubes at elevated temperatures, the presence of metallic particles reduces the temperature at which the nanotube material remains stable. This occurs because transition metals and transition metal compounds are known to catalyze the reaction of the nanotubes with other chemicals, such as oxygen, at elevated temperatures. High purity nanotubes, with the transition metal species substantially removed, would provide greater chemical stability to the nanotubes and a longer performance life to applications involving them.
The present invention relates to purified single-wall carbon nanotubes and means for their preparation. Known methods of single-wall carbon nanotube production all result in a product that contains single-wall carbon nanotube in addition to impurities such as particles of the metal catalyst used in single-wall carbon nanotube production and small amounts of amorphous carbon sheets that surround the catalyst particles and appear on the sides of the single-wall carbon nanotubes and xe2x80x9cropesxe2x80x9d of single-wall carbon nanotubes produced. The present process for purification comprises oxidizing the single-wall carbon nanotube material in an oxidizing gaseous atmosphere and treating the material with an aqueous solution of a halogen-containing acid. The oxidation step can be performed in a dry atmosphere or in an atmosphere comprising water vapor. The oxidation is performed at a temperature of at least about 200xc2x0 C. In one embodiment, the process also includes repetition of the oxidizing and halogen-containing acid treating cycle at the same or at higher subsequent oxidizing temperatures.
Many processing methods and articles of manufacture involving single-wall carbon nanotube are enhanced by the use of pure single-wall carbon nanotube material in which the presence of such impurities is minimized. There is a clear need for pure nanotube material and methods for its production. The present invention relates to such high-purity nanotube material, specific means for the production of the material, and articles of manufacture incorporating said material. The purification methods disclosed involve gas and liquid phase chemical treatments of the as-produced single-wall carbon nanotube material. The methods disclosed are scalable to provide for large amounts of high purity single-wall carbon nanotube material.
The material of this invention provides a superior nanotube material for applications because of the relative absence of impurities of metal and amorphous carbon, both of which exhibit different chemical, physical and electrical behavior than the single-wall carbon nanotube themselves.
Single-wall carbon nanotubes, purified to remove residual metal, are more stable and resistant to chemical attack at temperatures where metal-containing nanotube material would be chemically reactive. One example of such chemical attack would be oxidation of the nanotube material in air at temperatures exceeding 200xc2x0 C. Without being bound by theory, it is believed that the presence of transition metal impurities in the nanotube material catalyzes the oxidation of that material; and removal of the transition metal impurities would increase the oxidation temperature of nanotube material in air to over 600xc2x0 C. The present invention enables the chemical processing of nanotubes by providing high purity single-wall carbon nanotubes for reactions with other chemicals to produce new materials and devices. The present invention also enables aggregation of the single-wall carbon nanotubes into ropes and networks and therefore enables all articles of manufacture, materials and processes that depend on the propensity of single-wall carbon nanotubes and ropes of single-wall carbon nanotubes to aggregate. Such entities include composite materials comprising single-wall carbon nanotubes wherein the materials"" electrical, mechanical, optical, and/or thermal properties are enhanced by the presence of nanotube networks within the material. Such materials include bulk composite materials, paints, coatings, and adhesives, whose electrical, mechanical, optical, and/or thermal properties depend in part on the presence of nanotube networks therein, electrical circuitry, electronic devices (including batteries, capacitors, transistors, memory elements, current control elements, and switches) whose properties and function depend in part on the presence of nanotube networks therein.